The present invention relates to a heat type air flow meter, especially relates to a heat type air flow meter which is improved so as to suppress radio wave interference.
As an improved method for suppressing radio wave interference in a heat type air flow meter, a high frequency band cut-off filter is provided between both bodies of a heating resistance body and a heat sensitive resistance body and the temperature control circuit as described in Japanese Patent Application Laid-open 2-124429(1990).
The cause of a malfunction of the heat type air flow meter under high frequency and strong electric field is the high frequency electric current induced in the both resistances of the heat generation resistance and heat sensitive resistance, and as both resistance elements are covered on metal body having shield effect generally, even the method mentioned in the Japanese Patent Laid-Open issue 2-124429 was not able to fully suppress the radio wave interference.
The air passage body is today made of resin, however, and therefore the above-mentioned method cannot be used. Then, as high frequency band cut-off filter is generally used, the high frequency noise which is induced in both resistance bodies invades the control circuit with high frequency band cut-off filter mentioned above is taken in.
Furthermore, the high frequency band cut-off filter mentioned above is provided on a signal ground. Consequently, there is a danger that the high frequency noise acts on a common mode of the control circuit and causes false movements.